3202. Roadblocks

单点时限: 2.0 sec

内存限制: 256 MB

Bessie has moved to a small farm and sometimes enjoys returning to visit one of her best friends. She does not want to get to her old home too quickly, because she likes the scenery along the way. She has decided to take the second-shortest rather than the shortest path. She knows there must be some second-shortest path.

The countryside consists of $R$ $(1 \leq R \leq 100~000)$ bidirectional roads, each linking two of the $N$ $(1 \leq N \leq 5000)$ intersections, conveniently numbered $1$..$N$. Bessie starts at intersection $1$, and her friend (the destination) is at intersection $N$.

The second-shortest path may share roads with any of the shortest paths, and it may backtrack i.e., use the same road or intersection more than once. The second-shortest path is the shortest path whose length is longer than the shortest path(s) (i.e., if two or more shortest paths exist, the second-shortest path is the one whose length is longer than those but no longer than any other path).


  • Line $1$: Two space-separated integers: $N$ and $R$
  • Lines $2$..$R+1$: Each line contains three space-separated integers: $A$, $B$, and $D$ that describe a road that connects intersections $A$ and $B$ and has length $D$ $(1 \leq D \leq 5000)$


The length of the second shortest path between node $1$ and node $N$


4 4
1 2 100
2 4 200
2 3 250
3 4 100


Two routes: 1 -> 2 -> 4 (length $100+200=300$) and 1 -> 2 -> 3 -> 4 (length $100+250+100=450$)

14 人解决,16 人已尝试。

23 份提交通过,共有 65 份提交。

4.7 EMB 奖励。

创建: 5 年,7 月前.

修改: 5 年,1 月前.

最后提交: 8 月,2 周前.

来源: USACO 2006 November Gold